1. Field of the Invention
The present invention relates to ultrasonic transmitters-receivers and, more particularly, to acoustic transducers.
2. Description of the Prior Art
An acoustic transducer performs the operation of converting electrical signals to acoustic signals and/or the inverse operation. Acoustic transducers are presently used in a variety of systems that measure distance or velocity with sound waves. These systems include underwater sonar and medical imaging equipment. In addition to measurement functions, acoustic transducers are used in underwater communications systems, including signal detectors-classifiers. Medical treatment devices, such as those used for the destruction of tumors, also make use of acoustic transducers. For these applications, the acoustic transducers operate at ultrasonic frequencies, i.e., frequencies above the upper limit of human hearing.
As an example of an acoustic transducer application, in many underwater sonar systems, velocity measurements are made using the principle of the Doppler shift. One type of Doppler sonar system is a current profiler. Typically, current profilers are used to measure current velocities in a vertical column of water for each depth "cell" of water up to a maximum range, thus producing a "profile" of water velocities. The general profiler system includes one or more acoustic transducers to generate pulses of sound (which when downconverted to human hearing frequencies sound like "pings") that backscatter as echoes from plankton, small particles, and small-scale inhomogeneities in the water. The received sound has a Doppler frequency shift proportionate to the relative velocity between the scatters and the transducer.
The physics for determining a single velocity vector component (v.sub.x) from such a Doppler frequency shift may be concisely stated by the following equation: ##EQU1## In equation (1), c is the velocity of sound in water, which is about 1500 meters/second. Thus, by knowing the transmitted sound frequency, f.sub.T, and declination angle of the transmitter transducer, .theta., and measuring the received frequency from a single pulse, the Doppler frequency shift, f.sub.D, determines one velocity vector component. By adding more transducers, additional components of velocity are measured.
A pulse may comprise one or more cycles of a reference frequency. Profilers characterized by one common type of processing use the echoes from each pulse independently, measuring phase changes over a fraction of the pulse duration to determine the Doppler frequency shift, i.e., f.sub.D =.theta./T, where .theta. is a phase change calculated from performing an autocorrelation on a received waveform and T is a measurement period.
Such systems estimate the Doppler shift from either the phase change per unit time or the shift in spectral peak of a single pulse echo. The transmitted waveform is typically a periodic pulse train characterized by a pulse repetition interval (PRI). Thus, to provide for a round-trip visit (including echo time) to the particles, or scatterers, in a given depth cell, the maximum profiling range or depth is one-half the PRI. The received echoes are placed in memory bins defined by "time-gating" the received signal, i.e., echoes received at time t.sub.n come from scatterers located at a distance 1/2ct.sub.n. The width of the gate is usually matched to the pulse length, T, giving a range resolution of 1/2cT. The velocity (v) of the scatterers in a particular cell is related to the Doppler shift f.sub.D by the following equation: EQU v=1/2.lambda.f.sub.D ( 2)
where .lambda. is the acoustic Wavelength (for example, .lambda.=0.5 cm at 300 kHz).
Thus, range and velocity resolutions are proportional to the wavelength of the transmitted acoustic signal. A shorter wavelength (higher frequency) will generally improve the accuracy of the spacial-temporal resolutions. However, a longer wavelength (lower frequency) is used to achieve a greater profiling range, or depth, since increased power is available at longer wavelengths. Therefore, no single reference frequency is appropriate for all applications.
Ordinarily, an acoustic signal, or sound wave, is propagated into the water by vibrating a piezoelectric disk, or plate, with an electrical signal; the electrical signal having the same reference frequency as the acoustic signal. The thickness of the piezoelectric plate determines the reference frequency (e.g., a thickness of 6 millimeters produces a frequency of 300 kHz). For a given frequency, the diameter (D) of the plate determines the beamwidth (b) according to the following equation: EQU b=.lambda./D (3)
Another consideration in designing acoustic transducers is improving the impulse response (or linearity of frequency transfer between the electrical and acoustic signals) to thereby minimize distortion. Since the impulse response of a system is directly related to the bandwidth, the criteria can be restated as finding a high-efficiency, broadband transducer. Piezoelectric ceramics, although well-recognized as having good impedance matching to electrical signals, also have impedances that are an order of magnitude higher than water, e.g., 33:1.5. It is well-known that bandwidth is narrowed by such impedance mismatching.
Broad bandwidth characteristics are extremely important in newer current profilers wherein multiple pulses are generated into the water "simultaneously" and, in addition, pulses may be modulated.
Researchers have sought to improve impedance matching between the acoustic source (e.g., piezoelectric plate) and the acoustic load, i.e., the medium of sound propagation. One important result by Desilets, et al. ("The Design of Efficient Broad-Band Piezoelectric Transducers", IEEE Transactions on Sonics and Ultrasonics, Vol. SU-25, No. 3, May, 1978, pp. 115-125) showed that impedances of matching layers (i.e., layering the piezoelectric plate with successively lower impedance materials) can be derived according to a binomial relationship already used for transmission lines. However, direct computation of the optimal, matched impedance design was found to be computationally intractable by Inoue, et al. ("Design of Ultrasonic Transducers with Multiple Acoustic Matching Layers for Medical Application", IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. UFFC-34, No. 1, January, 1987, pp. 8-15). Inoue, et al., nonetheless, made two simplifying assumptions to directly compute the optimal transducer design. First, all layer impedances were specified to be in monotonically decreasing order from the acoustic source to the acoustic load. Second, the ith matching layer thickness (t.sub.i) was derived using a common coefficient a.sub..lambda., as presented in the following equation: EQU t.sub.i =a.sub..lambda. .lambda..sub.i /4 (4)
where .lambda..sub.i is the ith matching layer wavelength of the reference frequency.
It has been found, however, that the Inoue, et al., results are not directly transferable to the operating parameters of current profilers. Specifically, medical applications, as indicated by Inoue, et al., typically operate at frequencies above 1 MHz. In contrast, typical frequencies of current profilers are in the range of 50 kHz to somewhat over 1 MHz. Since piezoelectric plate size is a function of frequency (lower frequency transducers requiring larger plates), much larger plate diameters are typically required for current profiler transducers than for medical imaging transducers.
Moreover, current profiler environments are subject to extreme temperature variations. For example, inside the arctic circle, a current profiler transducer can be transferred from air temperatures of -55.degree. C. to ocean water temperatures of 2.degree. C. Just transport of current profilers by air cargo subjects the transducers to temperatures of -40.degree. C.
At the other extreme, temperatures in the Indian Ocean and Red Sea are known to reach 40.degree. C. for several months of the year. At equatorial latitudes, temperatures on the deck of a ship may reach 60.degree. C., and the current profiler can be suddenly transferred from the ship to deep water temperatures that are much cooler.
Under such plate size and temperature constraints, materials that are presently used in acoustic transducers have unacceptable coefficients of thermal expansion. Indeed, when a current profiler using present materials is deployed in an environment subject to wide temperature variations, the piezoelectric plate simply shatters. Thus, a need exists for broadband acoustic transducers, having operating frequencies under 2 Megahertz, that can withstand extreme temperature variations.